Response Of Ionization Chamber Research Articles

Monte Carlo Modeling of Ion Chamber Performance Using MCNP

Ion Chambers have a generally flat energy response with some deviations at very low (<100 keV) and very high (>2 MeV) energies. Some improvements in the low energy response can be achieved through use of high atomic number gases, such as argon and xenon, and higher chamber pressures. This work looks at the energy response of high pressure xenon-filled ion chambers using the MCNP Monte Carlo package to develop geometric models of a commercially available high pressure ion chamber (HPIC). The use of the F6 tally as an estimator of the energy deposited in a region of interest per unit mass, and the underlying assumptions associated with its use are described. The effect of gas composition, chamber gas pressure, chamber wall thickness, and chamber holder wall thicknesses on energy response are investigated and reported. The predicted energy response curve for the HPIC was found to be similar to that reported by other investigators. These investigations indicate that improvements to flatten the overall energy response of the HPIC down to 70 keV could be achieved through use of 3 mm-thick stainless steel walls for the ion chamber.

Measurements of spatial variations in response of ionization chambers

Measurements of the spatial variations in the response of three ionization chamber (IC) designs were tested as a function of chamber bias voltage, incident X-ray flux and fill gas. Two components of spatial variation are seen. When the ionization chambers are near saturation, spatial variations exist that are tied to the chamber geometry. While the response of some chambers is relatively flat, others show significant variation across the IC. These variations appear to be inherent in the response of each IC at saturation. When the chamber is far from saturation, large spatial variations in response are present when N(2) is used as a fill gas, but not when ambient air is used as a fill gas. These appear to be tied to space charge effects.

Response of thimble ionization chambers to high energy photon beams.

Response of thimble ionization chambers to high energy photon beams.

Detailed high‐accuracy megavoltage transmission measurements: A sensitive experimental benchmark of EGSnrc

There are three goals for this study: (a) to perform detailed megavoltage transmission measurements in order to identify the factors that affect the measurement accuracy, (b) to use the measured data as a benchmark for the EGSnrc system in order to identify the computational limiting factors, and (c) to provide data for others to benchmark Monte Carlo codes. Transmission measurements are performed at the National Research Council Canada on a research linac whose incident electron parameters are independently known. Automated transmission measurements are made on-axis, down to a transmission value of ∼1.7%, for eight beams between 10 MV (the lowest stable MV beam on the linac) and 30 MV, using fully stopping Be, Al, and Pb bremsstrahlung targets and no fattening filters. To diversify energy differentiation, data are acquired for each beam using low-Z and high-Z attenuators (C and Pb) and Farmer chambers with low-Z and high-Z buildup caps. Experimental corrections are applied for beam drifts (2%), polarity (2.5% typical maximum, 6% extreme), ion recombination (0.2%), leakage (0.3%), and room scatter (0.8%)-the values in parentheses are the largest corrections applied. The experimental setup and the detectors are modeled using EGSnrc, with the newly added photonuclear attenuation included (up to a 5.6% effect). A detailed sensitivity analysis is carried out for the measured and calculated transmission data. The developed experimental protocol allows for transmission measurements with 0.4% uncertainty on the smallest signals. Suggestions for accurate transmission measurements are provided. Measurements and EGSnrc calculations agree typically within 0.2% for the sensitivity of the transmission values to the detector details, to the bremsstrahlung target material, and to the incident electron energy. Direct comparison of the measured and calculated transmission data shows agreement better than 2% for C (3.4% for the 10 MV beam) and typically better than 1% for Pb. The differences can be explained by acceptable photon cross section changes of ≤0.4%. Accurate transmission measurements require accounting for a number of influence quantities which, if ignored, can collectively introduce errors larger than 10%. Accurate transmission calculations require the use of the most accurate data and physics options available in EGSnrc, particularly the more accurate bremsstrahlung angular sampling option and the newly added modeling of photonuclear attenuation. Comparison between measurements and calculations implies that EGSnrc is accurate within 0.2% for relative ion chamber response calculations. Photon cross section uncertainties are the ultimate limiting factor for the accuracy of the calculated transmission data (Monte Carlo or analytical).

SU-E-T-54: A Methodology for Monte Carlo Simulation Time Reduction in Dose Calculation for Reference Dosimetry

Purpose: To develop and optimize the simulation methodology to calculate the response of ionization chambers over a range of beam qualities commonly used in radiotherapy. The methodology can be used to determine the perturbation and beam quality correction factors for thimble ionizationchambers in high energy photons in a simple way. Methods: The Monte Carlo simulation code PENELOPE was used to simulate the dose in the sensitive volume of the NE2571 ionization chamber placed in a cubic water phantom, at the reference depth, for four different photon beams: 60Co, 4 MV, 6 MV and 10 MV. On a first step simulation, the radiation spectrum that reaches the geometric region of a 6 cm diameter sphere involving the chamber is sampled. Then, in a second simulation, the response of the ionization chamber is simulated using the sampled spectrum and without the cubic water phantom. The results of dose to the sensitive volume of the chamber in the two steps simulation was compared to the dose simulated in a one step simulation with the primary beam incident on the cubic phantom. Results: The dose values obtained without the water phantom, using the pre-sampled spectrum, are similar to the values obtained with those obtained in chamber positioned inside the phantom, with a maximum relative difference of 2.5% for the 10 MV beam. With the pre-sampled spectrum the time required for the simulation was about 30 times lower than for the one step simulation. The best results showed a relative difference of 0.4% for the 6 MV beam. Conclusions: The pre-sampling spectrum methodology proposed presented potential for significant simulation time reduction in dose calculations for reference dosimetry, studies of perturbation factors for thimble ionization chambers and beam quality correction determinations.

TH‐D‐BRCD‐01: TG‐51 Addendum: New KQ Values

The addendum to TG‐51, which is currently under review, will provide an expanded new set of beam quality conversion factors, kQ. The calculated values of kQ in the TG‐51 protocol are based on analytic calculations which make use of various tabulated values of measured and calculated factors uch as the water to air stopping‐power ratio, Pwall, Prepl and Pcel (for details see Ch 9 of the 2009 AAPM Summer School book, Med. Phys. Publishing, Madison, WI). However, with the development of the EGSnrc Monte Carlo system (Kawrakow, Med Phys 27(2000) 499) it is possible to accurately calculate the response of ion chambers. Also computers have became faster per unit cost and clever variance reduction techniques have been implemented for doing ion chamber calculations in complex geometries (Wulff et al, Med Phys 35(2008)1328). Hence ab initio Monte Carlo calculations of kQ have become both accurate and feasible using large clusters of computers. Values of kQ have been calculated this way for a total of 33 commonly used cylindrical ion chambers (Muir and Rogers, Med Phys 37(2010)5939) and these kQ values are accurately fit as a quadratic function of the beam quality specifier, %dd(10)x. Detailed estimates of the systematic uncertainty in these calculations have been made and range between 0.6% and 1.0%, depending on the assumptions made. The largest component is the uncertainty (previously 0.5%) in the assumed constancy with beam quality of (W/e)air, which relates the energy deposited in the cavity to the charge released in the air. In a detailed comparison of the calculated kQ values to the extensive high‐quality measurements by McEwen (Med Phys 37(2010)2179), Muir et al (Med Phys 38 (2011) 4600) found the mean percentage differences between the calculations and the experiments are 0.08(0.17), 0.07(0.32) and 0.23(0.31) in 6, 10 and 25 MV beams respectively (bracketed values are rms deviations). These discrepancies are well within the stated uncertainties of the measurements (about 0.3% to 0.4%) and the calculations (about 0.3% to 0.4% ignoring W/e uncertainties and assuming correlated uncertainties in photon cross section). These and similar comparisons for plane‐parallel chambers (Muir et al, Med Phys 39(2012) 1618) can be used to set an upper limit of 0.36% on the variation of (W/e)air with beam quality between 60Co and 25 MV beams (95% confidence). More importantly, the close agreement with experiment gives confidence in the accuracy of the Monte Carlo calculated values of kQ with a 68% confidence uncertainty of between 0.4% and 0.5%.The addendum to TG‐51 recommends use of these calculated values of kQ for the 21 ion chambers which meet the requirements for a reference class ion chamber.Learning Objectives1. To become aware of the new addendum to TG‐51 regarding reference dosimetry in photon beams.2. Understand how ab initio calculations of the TG‐51's beam quality conversion factor, kQ, are done3. Understand the uncertainties involved in ab initio calculations of kQ.

SU-E-T-146: Reference Dosimetry for Protons and Light-Ion Beams Based on Graphite Calorimetry.

The IAEA TRS-398 code of practice can be applied for the measurement of absorbed dose to water under reference conditions with an ionization chamber. For protons, the combined relative standard uncertainty on those measurements is less than 2% while for light-ion beams, it is considerably larger, i.e. 3.2%, mainly due to the higher uncertainty contributions for the water to air stopping power ration and the W air-value on the beam quality correction factors kQ,Q0 . To decrease this uncertainty, a quantification of kQ,Q0 is proposed using a primary standard level graphite calorimeter. This work includes numerical and experimental determinations of dose conversion factors to derive dose to water from graphite calorimetry. It also reports on the first experimental data obtained with the graphite calorimeter in proton, alpha and carbon ion beams. Firstly, the dose conversion has been calculated with by Geant4 Monte-Carlo simulations through the determination of the water to graphite stopping power ratio and the fluence correction factor. The latter factor was also derived by comparison of measured ionization curves in graphite and water. Secondly, kQ,Q0 was obtained by comparison of the dose response of ionization chambers with that of the calorimeter. Stopping power ratios are found to vary by no more than 0.35% up to the Bragg peak, while fluence correction factors are shown to increase slightly above unity close to the Bragg peak. The comparison of the calorimeter with ionization chambers is currently under analysis. For the modulated proton beam, preliminary results on W air confirm the value recommended in TRS-398. Data in both the non-modulated proton and light-ion beams indicate higher values but further investigation of heat loss corrections is needed. The application of graphite calorimetry to proton, alpha and carbon ion beams has been demonstrated successfully. Other experimental campaigns will be held in 2012. This work is supported by the BioWin program of the Wallon Government.

Investigation of thermal and temporal responses of ionization chambers in radiation dosimetry

The ionization chamber is a primary dosimeter that is used in radiation dosimetry. Generally, the ion chamber response requires temperature/pressure correction according to the ideal gas law. However, this correction does not consider the thermal volume effect of chambers. The temporal and thermal volume effects of various chambers (CC01, CC13, NACP parallel-plate, PTW) with different wall and electrode materials have been studied in a water phantom. Measurements were done after heating the water with a suitable heating system, and chambers were submerged for a sufficient time to allow for temperature equilibrium. Temporal results show that all chambers equilibrate quickly in water. The equilibration time was between 3 and 5 min for all chambers. Thermal results show that all chambers expanded in response to heating except for the PTW, which contracted. This might be explained by the differences in the volumes of all chambers and also by the difference in wall material composition of PTW from the other chambers. It was found that the smallest chamber, CC01, showed the greatest expansion. The magnitude of the expansion was ~1, 0.8, and 0.9% for CC01, CC13, and parallel-plate chambers, respectively, in the temperature range of 295-320 K. The magnitude of the detected contraction was <0.3% for PTW in the same temperature range. For absolute dosimetry, it is necessary to make corrections for the ion chamber response, especially for small ion chambers like the CC01. Otherwise, room and water phantom temperatures should remain within a close range.

Accuracy of the electron transport inmcnp5and its suitability for ionization chamber response simulations: A comparison with theegsnrcandpenelopecodes

In this work, accuracy of the mcnp5 code in the electron transport calculations and its suitability for ionization chamber (IC) response simulations in photon beams are studied in comparison to egsnrc and penelope codes. The electron transport is studied by comparing the depth dose distributions in a water phantom subdivided into thin layers using incident energies (0.05, 0.1, 1, and 10 MeV) for the broad parallel electron beams. The IC response simulations are studied in water phantom in three dosimetric gas materials (air, argon, and methane based tissue equivalent gas) for photon beams ((60)Co source, 6 MV linear medical accelerator, and mono-energetic 2 MeV photon source). Two optional electron transport models of mcnp5 are evaluated: the ITS-based electron energy indexing (mcnp5(ITS)) and the new detailed electron energy-loss straggling logic (mcnp5(new)). The electron substep length (ESTEP parameter) dependency in mcnp5 is investigated as well. For the electron beam studies, large discrepancies (>3%) are observed between the MCNP5 dose distributions and the reference codes at 1 MeV and lower energies. The discrepancy is especially notable for 0.1 and 0.05 MeV electron beams. The boundary crossing artifacts, which are well known for the mcnp5(ITS), are observed for the mcnp5(new) only at 0.1 and 0.05 MeV beam energies. If the excessive boundary crossing is eliminated by using single scoring cells, the mcnp5(ITS) provides dose distributions that agree better with the reference codes than mcnp5(new). The mcnp5 dose estimates for the gas cavity agree within 1% with the reference codes, if the mcnp5(ITS) is applied or electron substep length is set adequately for the gas in the cavity using the mcnp5(new). The mcnp5(new) results are found highly dependent on the chosen electron substep length and might lead up to 15% underestimation of the absorbed dose. Since the mcnp5 electron transport calculations are not accurate at all energies and in every medium by general clinical standards, caution is needed, if mcnp5 is used with the current electron transport models for dosimetric applications.

A convolution model for obtaining the response of an ionization chamber in static non standard fields

This work contains an alternative methodology for obtaining correction factors for ionization chamber (IC) dosimetry of small fields and composite fields such as IMRT. The method is based on the convolution/superposition (C/S) of an IC response function (RF) with the dose distribution in a certain plane which includes chamber position. This method is an alternative to the full Monte Carlo (MC) approach that has been used previously by many authors for the same objective. The readout of an IC at a point inside a phantom irradiated by a certain beam can be obtained as the convolution of the dose spatial distribution caused by the beam and the IC two-dimensional RF. The proposed methodology has been applied successfully to predict the response of a PTW 30013 IC when measuring different nonreference fields, namely: output factors of 6 MV small fields, beam profiles of cobalt 60 narrow fields and 6 MV radiosurgery segments. The two-dimensional RF of a PTW 30013 IC was obtained by MC simulation of the absorbed dose to cavity air when the IC was scanned by a 0.6 × 0.6 mm(2) cross section parallel pencil beam at low depth in a water phantom. For each of the cases studied, the results of the IC direct measurement were compared with the corresponding obtained by the C/S method. For all of the cases studied, the agreement between the IC direct measurement and the IC calculated response was excellent (better than 1.5%). This method could be implemented in TPS in order to calculate dosimetry correction factors when an experimental IMRT treatment verification with in-phantom ionization chamber is performed. The miss-response of the IC due to the nonreference conditions could be quickly corrected by this method rather than employing MC derived correction factors. This method can be considered as an alternative to the plan-class associated correction factors proposed recently as part of an IAEA work group on nonstandard field dosimetry.

Transferring calibration coefficients from ionisation chambers used for diagnostic radiology to transmission chambers

In this work, the response of a double volume transmission ionisation chamber, developed at the Instituto de Pesquisas Energéticas e Nucleares, was compared to that of a commercial transmission chamber. Both ionisation chambers were tested in different X-ray beam qualities using secondary standard ionisation chambers as reference dosimeters. These standard ionisation chambers were a parallel-plate and a cylindrical ionisation chambers, used for diagnostic radiology and mammography beam qualities, respectively. The response of both transmission chambers was compared to that of the secondary standard chambers to obtain coefficients of equivalence. These coefficients allow the transmission chambers to be used as reference equipment.

Comparison of secondary neutron dose in proton therapy resulting from the use of a tungsten alloy MLC or a brass collimator system

To apply the dual ionization chamber method for mixed radiation fields to an accurate comparison of the secondary neutron dose arising from the use of a tungsten alloy multileaf collimator (MLC) as opposed to a brass collimator system for defining the shape of a therapeutic proton field. Hydrogenous and nonhydrogenous ionization chambers were constructed with large volumes to enable measurements of absorbed doses below 10(-4) Gy in mixed radiation fields using the dual ionization chamber method for mixed-field dosimetry. Neutron dose measurements were made with a nominal 230 MeV proton beam incident on a closed tungsten alloy MLC and a solid brass block. The chambers were cross-calibrated against a (60)Co-calibrated Farmer chamber in water using a 6 MV x-ray beam and Monte Carlo simulations were performed to account for variations in ionization chamber response due to differences in secondary neutron energy spectra. The neutron and combined proton plus γ-ray absorbed doses are shown to be nearly equivalent downstream from either a closed tungsten alloy MLC or a solid brass block. At 10 cm downstream from the distal edge of the collimating material the neutron dose from the closed MLC was (5.3 ± 0.4) × 10(- 5) Gy/Gy. The neutron dose with brass was (6.4 ± 0.7) × 10(- 5) Gy/Gy. Further from the secondary neutron source, at 50 cm, the neutron doses remain close for both the MLC and brass block at (6.9 ± 0.6) × 10(- 6) Gy/Gy and (6.3 ± 0.7) × 10(- 6) Gy/Gy, respectively. The dual ionization chamber method is suitable for measuring secondary neutron doses resulting from proton irradiation. The results of measurements downstream from a closed tungsten alloy MLC and a brass block indicate that, even in an overly pessimistic worst-case scenario, secondary neutron production in a tungsten alloy MLC leads to absorbed doses that are nearly equivalent to those seen from brass collimators. Therefore, the choice of tungsten alloy in constructing the leaves of a proton MLC is appropriate, and does not lead to a substantial increase in the secondary neutron dose to the patient compared to that generated in a brass collimator.

Partial volume characteristics of ionization chambers in kilovoltage x-ray exposure measurements

AbstractThe partial volume (spatial) response of four ionization chambers (Keithley) in kilovoltage X-ray beams, generated by the Philips Super 80CP X-ray unit, was assessed. The volume of the ionization chambers were of 10 cm3, 15 cm3, 150 cm3, and 600 cm3 used with Keithley electrometer Model 35040. The beam output was measured using a monitor chamber (Radcal 6.0 cm3) placed close to the collimator. The source to chamber distance was kept constant at 1 m. For the measurement of the response of ionization chambers of 15 cm3, 150 cm3, and 600 cm3, a slit of 2.0 mm width was made in a lead sheet of 3.2 mm thick and size of 30 × 30 cm2 and was placed on the ionization chamber. The measurements were made for 81 kVp, 400 mA, and 0.25 s and the slit was moved at an increment of 2.0 mm over the entire length of the chamber. For the measurements of the ionization chamber of 10 cm3 (CT chamber), the beams of 120 kVp, 200 mA and 0.2 s were generated, and a slit of 5 mm width was made in a similar lead sheet that was moved at an increment of 5.0 mm. From the result it appears that the sensitive volumes of the ionization chambers affect the response of the ionization chamber to incident radiation.

A practical implementation of the 2010 IPEM high dose rate brachytherapy code of practice for the calibration of 192Ir sources

This paper details a practical method for deriving the reference air kerma rate calibration coefficient for Farmer NE2571 chambers using the UK Institute of Physics and Engineering in Medicine (IPEM) code of practice for the determination of the reference air kerma rate for HDR 192Ir brachytherapy sources based on the National Physical Laboratory (NPL) air kerma standard. The reference air kerma rate calibration coefficient was derived using pressure, temperature and source decay corrected ionization chamber response measurements over three successive 192Ir source clinical cycles. A secondary standard instrument (a Standard Imaging 1000 Plus well chamber) and four tertiary standard instruments (one additional Standard Imaging 1000 Plus well chamber and three Farmer NE2571 chambers housed in a perspex phantom) were used to provide traceability to the NPL primary standard and enable comparison of performance between the chambers. Conservative and optimized estimates on the expanded uncertainties (k = 2) associated with chamber response, ion recombination and reference air kerma rate calibration coefficient were determined. This was seen to be 2.3% and 0.4% respectively for chamber response, 0.2% and 0.08% respectively for ion recombination and 2.6% and 1.2% respectively for the calibration coefficient. No significant change in ion recombination with source decay was observed over the duration of clinical use of the respective 192Ir sources.

SU‐E‐T‐313: Ionization Chamber Measurements in a Small, Non‐Uniform Beam ‐ Applications for Synchrotron Beam Dosimetry

Purpose: The aim of this ongoing work is to develop a dosimetric method suitable for the biomedical beamlines at the Canadian Light Source (CLS), which offer novel imaging and therapy modalities. As a step in this process, we analyzed the response of a clinical, cylindrical ionization chamber to small, non‐uniform synchrotron beams to determine absolute dose rates in air based on clinical calibration factors obtained under broad‐beam conditions.Methods: Three detectors (Gafchromic EBT2 film, Capintec PR06C ion chamber, and PTW diamond detector) were used to determine the vertical profile of the filtered beam produced by the CLSˈs bending magnet beamline. The radial response function of the ion chamber was determined using a 100 micrometer slit beam. Matlab code incorporated this information to predict the relative ion chamber output as a function of nominal vertical beam collimation (0.1 – 8.7 mm) for both stationary and scanning ion chamber geometries. The beam‐size dependent output was also measured directly with the ion chamber.Results: The beam profile was Gaussian in shape, and measurements with all three detectors were in substantive agreement. The ion chamber response function showed the walls and inner electrode of the chamber. Predicted and measured values for the ion chamber response with varying field size were in excellent agreement for both the scanning and stationary measurements, validating the MATLAB algorithm. Based on the algorithm and using an interpolated clinical calibration factor, the dose rates for the filtered bending magnet beam ranged from 1.7 – 1.9 Gy/s at the maximum CLS storage ring current Conclusions: The excellent agreement between the predicted and measured results provides confidence that the developed model of ion chamber response to the small, non‐uniform beam is valid and suggests that absolute dose rates can be measured accurately using clinical ion chambers.

SU-E-T-98: VMAT Quality Assurance: Improving the Angular Correction Factors for Ion Chamber Arrays

Purpose: Two‐dimensional (2D) detector arrays for QA of volumetric‐modulated‐arc‐therapy (VMAT) plans show angular dependence. An angular correction‐factor‐table (CFs) minimize this effect, but assumes: (1) the response of all ion chambers (IC) is identical per angle, (2) the ICA response from 0–180° = 180–360°, and (3) the ICA response is independent of the direction of rotation. We investigated whether these assumptions are correct and developed improved CFs. Methods: Measurements were done with a 2D ICA (Matrixx®, IBA Dosimetry). Dose was calculated with the AAA algorithm (Eclipse v8.6, Varian Medical Systems). Default angular CF tables were available for 0–180° and 0–360°. Custom CFs were created by dividing the calculated dose to each IC by the measured dose from static open‐fields delivered every 5°, for both coronal (couch included) and 2 sagittal (couch avoided) orientations. The CFs were tested with open‐field arcs and VMAT plans, comparing results using gamma analysis (3%, 3mm). Additionally, measurements of 1cm‐wide fields delivered clockwise (CW) vs. counter‐clockwise (CCW) were compared to examine effects of ICA structure. Results: The angular response of individual IC significantly varies (1stdev<=4.6%). The response from 0–180° vs. 180–360° is significantly different (paired t‐test, p<0.0001). Custom CFs generated greater agreement between measurement and calculation for open‐field arcs and VMAT plans than the default CFs. Custom CFs also improved agreement compared to no CF for open‐field arcs, but not for VMAT plans. The CW vs. CCW tests demonstrated that the dose in the penumbra region of narrow fields is affected by the direction of rotation (<=1%); the cumulative effect of this may be significant for VMAT. Conclusion: Custom CF tables, using measurements from 0–360° and individual chamber CFs rather than a global CF per angle, can improve the measurement accuracy of an ICA. Even with these thorough corrections, discrepancies were still observed for VMAT plans.

SU-E-T-87: Radiation Detector Responses to Applied Homogeneous Magnetic Fields

Purpose: To determine the relative dose deposition in a diamonddetector and an ion chamber from a clinical photon beam in an environment having varying magnetic fields, in an effort to evaluate dosimetry techniques in an integrated MR‐linac system. Methods: Using the Monte Carlo code PENELOPE, the PTW 60003 diamonddetector was modeled and irradiated with a 6MV photon beam spectrum (Varian 600C) in the presence of a homogeneous magnetic field. The distance from the source to the centre of the diamonddetector active volume was 100 cm and the detector was oriented perpendicular to the homogeneous magnetic field. An ion chamber (PR06) was also simulated with its long axis both parallel and perpendicular to the impinging photon beam, but perpendicular to the magnetic field in both cases. The homogeneous magnetic field was varied from 0 to 2500 gauss. The above system was also replicated experimentally with the aid of an electromagnet, and the measured relative ion chamber response as a function of magnetic field was compared with the simulations. Results: There is a significant variation (1.8% at 2170 Gauss) in the relative response of ion chamber as a function of magnetic field. Simulated ion chamber response follows the measured response closely. In contrast, both simulated and measured results indicate that the diamonddetector is relatively insensitive to the magnetic field compared to the farmer type ion chambers. Conclusions: This work has significant impact on dosimetry protocols for integrated MR‐linac systems, where the response of ion chambers is significantly affected by the presence of magnetic field. Thus, the use of ion chamber for reference dosimetry will require magnetic field dependent correction while a diamonddetector can safely be used as a beam scanning device.

SU-E-T-219: Ion Chamber Dosimetry Modification under Strong Magnetic Field Conditions; Air and Liquid Filled Chambers Study

Purpose: To examine the dose response of ion chambers in an external high magnetic field,conductingmeasurements and Monte Carlo simulations. To examine a liquid filled ion chamber (LION) as a low B field dependant on‐time dosimeter. Methods: This work investigates the response of a 0.6cm3 Farmer chamber to a 11.17 mCi 137Cs source under the influence of a 1.5 T magnetic field. A special setup allowed measurements for transverse and longitudinal field directions inside a 1.5T MRI bore. Monte Carlo simulations were conducted using FLUKA, a particle physics transport package. A PTW 0.002 cm3 MicroLion chamber is offered as a non‐correctional on‐line B field measurement alternative, since it introduces a small cavity and no air phantom voids. Results: Higher Farmer chamber currents were obtained when a transverse (to chamber central axis) magnetic field was applied, compared to zero B field conditions. Lower readings were obtained when a longitudinal field was present. Measurements have shown that the electric field between the electrodes has a very small impact on the ion collection, and can be neglected for simulation purposes. A microLion chamber was found not to have response impact due to the presence of a magnetic field. Conclusions: The dose response of a Farmer chamber in an external magnetic field varies with the average charged particle trajectory length. The average trajectory depends on the B field magnitude and direction, and on the energy spectrum of the primary photon beam. Thus, an alignment with the B field direction would be essential. A liquid‐filled chamber is offered as an alternative due to its homogeneous structure and small cavity that results in no B field correction needed.

TU-C-BRA-01: Progress in Calculations of KQ for TG-51

The calculated values of kQ in the TG-51 protocol are based on analytic calculations which make use of various tabulated values of measured and calculated factors such as the water to air stopping-power ratio, Pwall, Prepl and Pcel (for details see Ch 9 of the 2009 AAPM Summer School book, Med. Phys. Publishing, Madison, WI). Since the publication of TG-51 there have been improved calculations of almost all the factors required in TG-51. This talk will briefly review those improved calculations which are based on Monte Carlo calculations of ratios of the dose to the cavity of the ion chamber. However, with the development of the EGSnrc Monte Carlo system (Kawrakow, Med Phys 27(2000) 499) it became possible to accurately calculate the response of ion chambers, not just the ratios of the dose to the cavity. This made it possible to accurately do ab initio Monte Carlo calculations of kQ. As computers became faster per unit cost and clever variance reduction techniques for doing ion chamber calculations in complex geometries were implemented (Wulff et al, Med Phys 35(2008)1328) these calculations became both accurate and feasible (albeit with large clusters of computers). Values of kQ have been calculated this way for a total of 33 commonly used cylindrical ion chambers (Muir and Rogers, Med Phys 37(2010)5939). Detailed estimates of the systematic uncertainty in these calculations have been made and range between 0.6% and 1.0%, depending on the assumptions made. The largest component is the uncertainty (0.5%) in the assumed constancy with beam quality of (W/e)air, which relates the energy deposited in the cavity to the charge released in the air. In a detailed comparison of the calculated kQ values to the extensive high- quality measurements by McEwen (Med Phys 37(2010)2179), Muir et al (submitted, 2011) found the mean percentage differences between the calculations and the experiments are 0.08(0.17), 0.07(0.32) and 0.23(0.31) in 6, 10 and 25 MV beams respectively (bracketed values are rms deviations). These discrepancies are well within the stated uncertainties of the measurements (about 0.3% to 0.4%) and the calculations (about 0.3% to 0.4% ignoring W/e uncertainties and assuming correlated uncertainties in photon cross section). These comparisons can be used to set an upper limit of 0.4% on the variation of (W/e)air with beam quality between 60Co and 25 MV beams (95% confidence). More importantly, the close agreement with experiment gives confidence in the accuracy of the Monte Carlo calculated values of kQ with a 68% confidence uncertainty of between 0.4% and 0.5%. Learning Objectives 1. Understand the basis of calculated kQ values in the original TG-51 protocol 2. Become aware of the improvements made in many of the required correction factors in the last decade 3. Understand how ab initio calculations of kQ are done 4. Understand the uncertainties involved in ab initio calculations of kQ

Monte Carlo simulation for energy deposition of an ionisation chamber based on the equivalent electron source theory and experimental verification for the theory

The main research described in this paper includes three sections. First, research on the response of the stainless steel ball-shaped ionisation chamber by experimental methods. Secondly, calculation of the response of the chamber with the general Monte Carlo EGS4 code in order to compare with the equivalent electron source theory by calculation methods. Finally, calculation of the response of the ionisation chamber with the equivalent electron source theory. The results show that the calculated results of the equivalent electron source theory coincide very well with those of the experiments when the atomic number of the chamber wall is close to one of the gases (such as Ar and Kr), and the calculated results coincide with those of the experiments to a certain extent when the atomic number of the chamber wall is not close to one of the gases (such as He and Xe).